Recording velocity time history of highspeed projectiles created by shockwaves, high energy plasmas and other rapid-acceleration devices is a major challenge for explosives and shock dynamic research labs, all over the world. The velocity of such fast moving projectiles ranges typically from 1-100 km/sec and is associated with large acceleration and retardation. The total time for recording velocity time history of such rapid moving surfaces is no more than a few micro seconds putting major constraints on the response time of measurement systems. Important optical techniques for measuring velocity under such extreme conditions, often encountered in dynamic compression research, include active shock breakout technique, inclined mirror measurement method and laser based interferometry. In active shock breakout, changes in the reflected light intensity generates signal for arrival of a mechanical wave whereas the inclined mirror measurement approach is based on the changes in reflected light direction for information of velocity of a fast moving surface. Both these techniques, however, provide only an average velocity of the projectile and offer limited accuracy. Laser based interferometry is the most versatile velocity measurement technique which is based on the measurement of changes in the optical phase of light reflected from the moving surface.
Interferometric techniques, besides being non-intrusive in nature, provide complete velocity time history, thereby enabling measurement of large acceleration/retardation along the projectile flight. Moreover, interferometry techniques are far more accurate and offer better time resolution. Displacement and velocity interferometers are two prior art systems often used for the study of shock dynamic phenomena. In a displacement interferometer, the light that undergoes a Doppler shift due to its reflection from a moving object/projectile is interfered with a reference light (without Doppler shift) to measure the displacement of the object as a function of time. The received data is then differentiated with respect to time for obtaining velocity time history of the projectile. One major difficulty with this approach is that the Doppler shift obtained is, in general, very large, which imposes serious constraints on the bandwidth of detectors and measuring equipment. Moreover, displacement interferometers put stringent requirements on temporal and spatial coherence of the laser source and surface finish of the moving surface. Further, the movement of the surface should essentially be along the optical axis of the collection optics to obtain proper contrast in the fringe pattern.
In a velocity interferometer, the Doppler shifted light due to reflection from a moving target is made to interfere with a delayed version of it. As a result, the velocity interferometers do not require high degree of coherence of the laser source. Moreover, the quality of the fringe pattern and hence S/N ratio of system is not affected by the nature of the target surface, smooth or diffused, and by any tilt generated during movement of the target. Hence, the system is often referred to as velocity interferometer system for any reflector (VISAR). Today, VISAR is one of the most prominent diagnostic tools for velocity measurements of short duration, high speed motions generated in dynamic compression phenomena. Since its invention in 1972, VISAR has undergone many up-gradations and modifications to its original configuration. Push-pull form of VISAR is the most widely used and preferred choice among all available VISAR configurations. It provides better signal strength, higher rejection of unwanted self-light of the projectile with minimum constraints on coherence of the optical source and surface finish of the target. The push pull form of VISAR is described here, in brief, for the sake of completeness.
In a push-pull VISAR, light from a laser source is relayed to the target using discrete optical components or a fiber cable. The use of fiber cable reduces the alignment complexities and offers large stand-off distance. A VISAR probe connected to the other end of the fiber is used to focus the laser light onto the target. The light reflected off the target is received by the same probe and is relayed back using another optical fiber. The light received at the output of this fiber is collimated to form input to a push-pull VISAR. FIG. 1 depicts schematic of a push pull VISAR. The input beam is amplitude split using a beam splitter and resulting beams are made to traverse two different paths (legs) of the interferometer. A time delay between the two beams is introduced by placing a glass bar/etalon in one of the two legs of the interferometer. The light beams reflected from the end mirrors in the two legs are recombined using the same beam splitter. Difference in the travel time of the light beams in the two legs results in the formation of interference fringe pattern. When the light entering the interferometer is Doppler shifted because of projectile motion, it results in fringe shift proportional to the instantaneous velocity of the projectile. The changes in the fringe shift with time form the basis for recording velocity time history of the moving surface. The instantaneous velocity of the moving surface is given by the Eq. 1.
                              U          ⁡                      (            t            )                          =                              kF            ⁡                          (              t              )                                =                                    λ                              2                ⁢                τ                                      ⁢                          F              ⁡                              (                t                )                                                                        (        1        )            
Where k=λ/2τ is the velocity change that would result in shift of one fringe in the VISAR. This value, often referred to as velocity per fringe (VPF) constant and denoted by symbol k is inversely related to the delay time τ and hence to the length of the delay bar h i.e.
                    τ        =                                            2              ⁢              h                                      C              0                                ⁢                      (                          n              -                              1                n                                      )                                              (        2        )            where h is the length of delay bar and c0 is the velocity of light in free space. The velocity of the projectile at any instance is obtained from the product of VPF constant and fringe count F(t). Use of polarizing optics in the interferometer generates quadrature signals that allow measurement of fringe count with an accuracy of about ±2% of one fringe, thereby enabling very precise and accurate velocity measurements. Besides measurement accuracy, the quadrature technique helps in differentiating between acceleration and retardation encountered along the path of the projectile. Special feature of a push pull VISAR is that quadrature signals received on two sides (transmitted and reflected beam) of the beam splitter are 180 degree out of phase. Subtraction of corresponding quadrature voltage signals at output of detectors on either side of beam splitter helps in increasing the signal strength by nearly two fold, rejection of unwanted self-light of the projectile and simplification in processing of velocity measurement data.
Most of the VISAR systems employ a fixed length of etalon/delay bar. Due to fixed length of the etalon, the velocity-per-fringe constant and hence the sensitivity range of velocity measurements is fixed. In order to change sensitivity and time resolution of velocity measurements, which is a general requirement in the dynamic compression research, the etalons need to be added to or subtracted from the existing system. A VISAR may use multiple etalons with provision of addition and subtraction of etalons. The use of multiple etalons, however, demands additional hardware, increased cost and alignment complexities.
The limitations and disadvantages of conventional and traditional system are apparent to the person skilled in the art, including for example, a) the limitation of fixed sensitivity range and time resolution of velocity measurements and b) Requirement of additional hardware and alignment complexities. Hence, there exists a strong need to provide an effective and versatile velocity interferometer for any reflector, wherein length of the etalon can be changed in a continuous manner to change the sensitivity and time resolution of the velocity measurements, effectively without additional hardware and alignment complexities.